Crossing The Tacoma Narrows

Every Once in a While We Get to do Something Memorable. To a transmission line engineer, that memorable event may be designing an extraordinarily long transmission line with a great number of repetitive details to manage, or it may be designing a single span on which nothing is typical. Whatever form the memorable project takes, we all cherish the opportunity and hope for another. In 2003, Tacoma Power (Tacoma, Washington, U.S.) offered POWER Engineers Inc. (POWER; Hailey, Idaho, U.S.) such an opportunity. What ensued was three years of once-in-a-lifetime challenges and entertainment.

THE ORIGINAL CROSSING

In 1926, Tacoma Power constructed a 115-kV double-circuit crossing of the Puget Sound Narrows near Tacoma to bring power from its new Cushman Generating Station in the Olympic Mountains to the city. At the time, it was the longest transmission-line span in the world at 6240 ft (2 km), tower to tower. It was located about 1 mile (1.6 km) north of the location where the infamous “Galloping Girdy” Tacoma Narrows suspension bridge was built and failed in 1940.

The original crossing employed a design that had a mystique that preceded any engineer's actual encounter with it. Each circuit was supported on its own pair of 325-ft (100-m)-tall latticed-steel riveted, painted towers. At the tower tops, a large platform area provided access to all three-phase conductors that passed over the tower on large sheaves. While these 7-ft (2-m)-diameter wheels sat on bearings to allow unequal conductor tension across the towers to equalize, it is unlikely they ever rotated more than an inch or two during their 80-year service life.

On the long-span side of the towers, the conductors were broken by an in-line insulator bundle that isolated the conductor across the water from the portion of the cables that crossed over the towers' wheels and became back-span guys that anchored directly into concrete blocks on the ground about 1000 ft (304 m) behind the towers. In fact, the conductor was a 1.25-inch (3.2-cm)-diameter high-strength stranded-steel cable. On the backside of the insulator bundle, a pair of cables became the back-up guy for the single-cable conductor. Thus, each wheel was in fact a pair of grooved sheaves.

Attached to the insulator-bundle assembly and parallel to each conductor in the long span were two more cables that extended 70 ft and 90 ft (21 m and 27 m) into the span. These stress-relieving cables clamped to the crossing cable, so that each took up 22% of the tension in the main crossing cable.

Even though the crossing cables were installed to 22% of their breaking strength at a catenary constant of nearly 12,000 ft (3660 m) — twice the typical value — there were no vibration dampers on the crossing cables or the back-span guys. In 80 years, there had been no vibration problems with the crossing. Ingenious, indeed.

TIME FOR CHANGES

When the original crossing was constructed, the locale was wilderness. The city of Tacoma was over the horizon. Today, the right-of-way has been whittled away in places and some very nice homes have been constructed right up against the right-of-way fence. There are stories of ice falling off a structure onto the roof of the closest house. Needless to say, the location has changed in terms of a construction site. Certainly, over the years, the power capacity the steel-cable conductors can deliver has been met. Any planner's projection for the future suggested the real need to replace the conductors. The old conductors were rated at only 300 A (nominal) and 600 A (emergency) to allow full output of the Cushman generation on a single circuit.

The clincher for change was the result of a prolonged and detailed inspection of the 80-year-old towers for structural integrity. In short, the consensus was that they had more than fulfilled their service-life expectancy, and many more years of service were not realistic given the corrosion that had begun on some of the joints. By the time POWER was called to the table, Tacoma Power had decided that the towers needed to be replaced, and it made sense to replace the conductors at the same time.

UP FOR THE CHALLENGE

Tacoma Power decided to reconstruct the crossing using an engineer-procure-construct (EPC) contract. Under the terms of the contract, an owner's engineer (OE) on the team helps the owner develop and manage the plan and contract. POWER provided the OE service to Tacoma.

Tacoma Power and POWER were required to execute a series of design-option exercises for the purpose of identifying features for the crossing that were considered mandatory, possible/acceptable and unacceptable. Only after these exercises were completed could the team write technical specifications that would guide the EPC engineering team toward the desired features and away from unwanted, or unacceptable, features.

The design team studied crossing options with two circuits and three circuits. The team also looked at multi-circuit towers versus single-circuit towers, and tubular designs versus latticed-steel designs. This latter point was largely an aesthetics exercise. Each of the options was measured against project cost and schedule. The primary constraints on the project — other than the obvious need to manage cost — were the need for a one-year construction schedule and the need to keep at least one of the two existing circuits operating at all times during construction.

It became clear that this crossing would no longer be anything like the longest span in the world. The towers were not going to be nearly the tallest in the world, and the conductors were not going to be pulled nearly as tight as some of the tightest in the world. However, the combination of span length, conductor tension, tower height, right-of-way constraints, and schedule and outage limitations made the project very challenging.

CONDUCTOR COLLABORATION

About 3 miles (5 km) away from the crossing location there is a small municipal airport. Calculations by Federal Aviation Administration experts placed a limit on the west-side tower at 444 ft (135 m). We put a self-imposed height limit of 500 ft (150 m) on the east tower site. Although both the east and west towers sit on bluffs nearly 300 ft (90 m) high, the required 200-ft (60-m)-plus of navigable water clearance meant the conductors needed to be pulled to a catenary constant of nearly 11,000 ft (3350 m).

A review of the performance of other long, tight-crossing spans — primarily in Norway — showed that the parameters associated with this project regarding conductor-vibration activity placed it on the boundary between crossings that are trouble-free and those that develop damage from Aeolian vibration. It became readily apparent that attention to the conductor design and its vibration-mitigation design would be the most critical part of the project design.

The tower height limits and clearance requirements forced a maximum sag of about 430 ft (130 m). A design catenary constant of 11,500 ft (3500 m) and the National Electric Safety Code (NESC) limit of 25% of rated strength on final, everyday tension meant that the conductor needed a strength-to-weight ratio of 46,000 ft (14,000 m). The original all-steel cable conductor had a strength-to-weight ratio of 55,400 ft (16,890 m). Typical 26/7 ACSR stranded conductor has a ratio of 28,800 ft (8780 m). Thus, a special conductor with a high steel content was required.

About 50 years ago, a double-circuit 275-kV crossing of the Severn River in England was constructed. It immediately began to periodically gallop and flash over on summer days without the presence of ice. A study showed that the oblique 5400-ft (1650-m) crossing of the river exposed the round strands on the top and bottom surfaces of the conductors to different angles to the wind that ran up or down the river on a daily pattern. The different angle meant different roughness and different lift on the top surface versus the bottom surface of the conductors. Galloping ensued. The solution was to wrap the entire span of all conductors in tape to hide the strand roughness differences from the wind. The galloping stopped.

Although the original steel cables on the Narrows Crossing were galvanized, they were also painted with bitumen several times over to help protect them from the sea-air environment. The oblique angle of the crossing to the prevailing wind is practically identical to that of the Severn River Crossing. The Narrows Crossing showed no galloping activity because the strands were also hidden from the wind by the bitumen paint.

At this point, it is useful and honorable to pass along the sage advice offered by H. Brian White, a transmission line consultant, when we visited the site in the early days of our design exercises. “It is useful to understand why this crossing has been an 80-year success story, so that you incorporate those same features into the new crossing design,” said White. For this reason, a smooth-body conductor was considered necessary.

It was also necessary to boost the strength-to-weight ratio of the conductor by using alloyed aluminum. For these reasons, a conductor was designed by collaboration between the owner, the OE and the manufacturer, J-Power Systems Corp. of Japan. The final product was a 61-strand high-strength steel core wrapped in one inner layer of round-strand hard-alloy aluminum and one outer layer of trapezoidal-strand medium-alloy aluminum. The outer layer alloy was weakened to allow the forming of the trap strands. The outer diameter of the conductor is 1.42 inches (36 mm). The strength and weight are 147,100 lbs (66,700 kgs) and 2.727 lbs/ft (4.07 kgs/m), respectively, for a strength-to-weight ratio of 53,940 ft (16,400 m).

Reels of 9500 ft (2900 m) each were purchased along with compression end fittings and handed off to the EPC contractor for installation. No splices were allowed in the three spans between anchor towers. The conductor has 1001 kcmils of aluminum and an ampacity of about four times that of the original steel cable. Given that the design voltage for the project was 230 kV, the potential capacity of the crossing is greatly improved.

STANDING UP TO LOAD

The Puget Sound area is a relatively benign design environment. There are no tornados, there are limited lightning events, extreme winds are rare, temperatures are temperate, and ice is rare and modest. Selecting design parameters was not difficult, and the final controlling load cases revolved around high-wind events. Application of the wind to the tall towers required a detailed understanding of, and belief in, the ASCE Manual 74 methodologies. The tower heights and terrain factors were outside the domain of the recent and improved (complicated) NESC scope.

The investment in this crossing reconstruction amounted to about US$8 million per mile. It is an expensive 2-mile (3.2-km) patch of transmission line by normal standards. But if the span were to fail in any way, the cost of reinstatement and of damage repair would be much greater than that figure. Therefore, the primary objective of the design criteria was to protect the crossing against costly failure. Our focus was to manage the failure of a dead-end assembly or the dropping of a phase off the tall tower by the failure of a suspension assembly.

Every suspension and dead-end assembly was proof loaded to 50% of conductor strength before installation. Dead-end assemblies were designed to the strength of the conductor. Bundled-insulator assemblies were designed for the loss of one of the insulator units. While this might be a costly approach on a long line with hundreds of towers, it is prudent and not costly on an expensive 2-mile-long project.

DESIGNING THE TALL TOWERS

The EPC contractor was given the freedom to choose the tower design and style, including the option to incorporate space for a third circuit. The final choice was a double-circuit, latticed-tower design with six arms and a slightly flared body. The tower heights required that they be painted in pretty, government-issued bands of red and white. The final tower heights were 440 ft (135 m) and 475 ft (145 m). The new towers were located in the open space between the existing pairs of single-circuit towers. The new towers are higher than the original towers, because the new conductor sags are slightly greater than that of the originals, and the phases are stacked vertically on the new towers.

The towers carry the conductors in suspension, and the arm length increases slightly on each higher arm. This feature gives the towers a bit of a strange appearance to the trained eye. The objective was to prevent the failure of a suspension assembly or a misstep by a maintenance crew from dropping any of the 10-ton phases on and destroying the arm below. With this feature, a single-phase failure is not likely to propagate into a multiphase failure.

The anchor towers hold six conductors on each side of the crossing, and each conductor terminates on its own separate structure. Each of the 12 single-phase structures is placed such that the loss of its strain-assembly connection will not impact and jeopardize any other. Each dead-end structure is an in-line guyed, tubular pole on a concrete foundation. As with the suspension arrangement on the tall towers, any single-phase failure is unlikely to propagate into a multiphase failure. These features protect the expensive, tall towers from becoming victims of a hardware failure.

ANOTHER 80 YEARS

The one-of-two-circuits outage window offered by the operating people was open from May through August. Outside this time frame, outages were not appreciated. This constraint defined the construction process and the design itself. Foundation work and site preparations took place from February to May. In May, the new towers were erected by crane up to near the existing conductor heights. After that, a gin pole was dragged out of 50-year storage, buffed up and put to work to erect the top half of the towers.

Conductor stringing required reasonably high pulling loads, although the largest tension was required to remove the existing crossing cables. This load approached 40,000 lbs (18,200 kgs). Once both new circuits were in place, construction crews removed the old conductor cables and then the venerable old towers.

In the final assessment, a new crossing was in place, and the team is satisfied that it can provide another 80 years of service, if required. Just as an aside, we are not aware that anyone found the gold rivet said to have been placed in one of the old tower legs.

Bob Kirchmeier has worked for Tacoma Power since 1982 on a wide range of generation, transmission and distribution projects while also supervising the Protection, Substation and Communications Engineering groups. He is now project manager for Tacoma's Major Projects group, which is rebuilding the Narrows Crossing and working on several other large projects. Kirchmeier holds a BSEE degree from Seattle University and is a registered professional engineer in the state of Washington. bkirchme@cityoftacoma.org

Peter G. Catchpole is a senior project manager with POWER Engineers Inc. He has spent 15 of his 30-year career in T&D with POWER. He holds a civil engineering degree from Queen's University (Kingston, Ontario, Canada) and is a member of IEEE and CIGRÉ. His work covers projects in various countries around the world. He is a registered professional engineer. pcatchpole@powereng.com